pinctrl: nuvoton: wpcm450: Refactor MFSEL setting code

In preparation for the next patch, which makes the logic around
setting/resetting bits in MFSEL a little more complicated, move that
code to a new function

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20221105185911.1547847-2-j.neuschaefer@gmx.net


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>